1. Field of the Disclosure
This invention pertains to an apparatus and method for preparing a printing plate from a photosensitive element and, in particular, to an apparatus and method for exposing the printing form to actinic radiation.
2. Description of Related Art
Flexographic printing plates are well known for use in relief printing on a variety of substrates such as paper, corrugated board, films, foils and laminates. Flexographic printing plates can be prepared from photosensitive elements containing a layer of a photosensitive composition such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. Photosensitive compositions, which may be referred to as photopolymerizable compositions, generally contain an elastomeric binder, at least one monomer, and a photoinitiator. Photosensitive elements generally have the layer of the photopolymerizable composition interposed between a support and a cover sheet or multilayer cover element. Upon imagewise exposure of the photosensitive element to actinic radiation, photopolymerization of the photosensitive composition occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. The exposed element can be treated with a suitable solution or treated thermally to remove areas of the photopolymerizable layer that were not exposed which provides a printing relief suitable for use in flexographic printing.
Most commercial flat-bed exposure apparatuses include at least one bank of tubular lamps, which is formed from a plurality of light tubes that are arrayed to form a wall-like effect. The tubular lights are typically fluorescent lamps that emit ultraviolet radiation at or in the range of wavelengths necessary to cause photochemical reaction of the exposed portions of the photosensitive element. A plurality of light tubes is necessary in order to achieve the actinic radiation energy necessary for photopolymerization of the photosensitive element to occur. Some other commercial flat-bed exposure apparatuses include two banks of tubular lamps. At least for exposure of the photosensitive element, the two banks of light tubes are spaced apart, opposite and parallel the other, and the element which is supported on a glass bed is located between the banks. In some cases, the position of one or both banks of tubes may be adjusted to create an appropriate space between the banks to accommodate exposing photosensitive elements. In other cases, an upper bank of light tubes is used for imagewise exposure and/or back exposure of the photosensitive element; and, a lower bank of light tubes is used for light-finishing exposure and post exposure of the relief formed printing plate through the glass bed. In some instances the light-finishing and post exposures are conducted as one step.
In most commercial flat-bed exposure apparatuses the photosensitive element is supported on the glass bed that is surrounded by ports, which in combination with a transparent membrane or coversheet that covers the photosensitive element and a negative film or phototool, draws a vacuum to bring the negative film in intimate contact with the photosensitive element prior to imagewise exposure. Imagewise exposure through a negative film or phototool having image-bearing art-work that is held in intimate contact under vacuum to the photopolymerizable layer of the photosensitive element is often referred to as analog workflow. Analog workflow requires the preparation of the phototool, which is a complicated, costly and time-consuming process requiring separate processing equipment and chemical development solutions. In addition, the phototool may change slightly in dimension due to changes in temperature and humidity, and when used at different times or in different environments, may give different results, which can ultimately result in the mis-registration of multicolor images during printing. Use of a phototool also requires special care and handling when fabricating flexographic printing forms to ensure intimate contact is maintained between the phototool and photosensitive element. In particular, care is required in the placement of both the phototool and the photosensitive element in the exposure apparatus along with special materials, e.g., bleeder strips, to minimize air entrapment during creation of a vacuum to ensure intimate contact. Additionally care must be taken to ensure all surfaces of the photosensitive element, the phototool, and the transparent membrane, are clean and free of dust and dirt. Presence of such foreign matter can cause lack of intimate contact between the phototool and the photosensitive element as well as image artifacts.
Alternatively, imagewise exposure can be through an in-situ mask having radiation opaque areas and transparent areas that had been previously formed above the photopolymerizable layer, so called digital workflow. In most instances of digital workflow, the in-situ mask is formed from a radiation-opaque layer that is integral with the photosensitive element, and thus there is no need to draw vacuum to assure contact of the in-situ mask with the element prior to imagewise exposure. The photosensitive element can be imagewise exposed in the presence of atmospheric oxygen for conventional digital workflow as described in U.S. Pat. No. 5,262,275; U.S. Pat. No. 5,719,009; U.S. Pat. No. 5,607,814; U.S. Pat. No. 6,238,837; U.S. Pat. No. 6,558,876; U.S. Pat. No. 6,929,898; U.S. Pat. No. 6,673,509; U.S. Pat. No. 6,037,102; and U.S. Pat. No. 6,284,431. Alternatively, the photosensitive element can be imagewise exposed in an environment having an inert gas and a controlled amount of oxygen that is less than atmospheric oxygen for modified digital workflow, by placing the photosensitive element in an exposure enclosure or chamber of an exposure apparatus as described, for example, in U.S. Pat. No. 8,241,835. In one embodiment, the enclosure can be sealed from external environment (room conditions) and includes an inlet port for introducing the inert gas and optionally additional oxygen into the enclosure and an outlet port for purging the air that is initially in the enclosure. The conventional digital workflow and the modified digital workflow each provide particular relief structure in the resulting printing form that has specific characteristics and qualities to print desired quality images and graphic information for certain end-use applications. Another digital workflow method disclosed by Zwadlo in U.S. Pat. No. 7,279,254, includes forming a mask digitally in a thermal imaging layer of a separate film, laminating the film with the mask to the photosensitive element, and imagewise exposing the photosensitive element through the laminated mask film. As such, digital workflow because of its ease of use and desirable print performance has gained wide acceptance as a desired method by which to produce a flexographic printing form from the photosensitive element.
However with ever increasing demands on quality, the current state-of-the-art flexographic printing forms may not perform as desired and have trouble meeting the ever increasing demands on quality. Exposure times vary from a few seconds to a several minutes depending upon the output of the lamps, distance from the lamps, desired relief depth, and the thickness of the photosensitive element. Since the photosensitive element is exposed to actinic radiation at three different steps in its conversion to a relief printing form, which includes a back exposure through the support, and imagewise exposure through the mask, and a post-exposure and finishing exposure, it is particularly desirable to create and maintain uniform conditions in the exposure apparatus so that the photosensitive element experiences consistent environment and uniform distribution of actinic radiation during each of these exposures. If the actinic radiation energy impinging the photosensitive element is too low, polymerization reaction may not start at all or may not occur deep enough in the layer of the photosensitive material which impacts the shape of small raised printing elements of the relief image. If the actinic radiation energy impinging the photosensitive element is too high such that the exposure time becomes very short, the shape of the raised printing elements is also poor. The raised printing elements have a shoulder, which is a portion of the raised printing element that transitions from a flat printing area to a sidewall, which becomes too steep, and small dots or lines do not have sufficient base and can easily chip away during printing.
Typically exposure apparatuses include a plurality of lamp tubes in order to achieve the actinic radiation energy necessary for photopolymerization of the photosensitive element to occur. During exposure the plurality of lamps is often in very close proximity to the photosensitive precursor. Due to the number and proximity of the lamps to the photosensitive element, and duration of the exposure, the temperature of the photosensitive element during exposure can change, i.e., increase, during exposure. It is also desirable to maintain the temperature constant on the photosensitive element, and avoid relatively hot or cool regions of the photosensitive element. Temperature changes of the photosensitive element, particularly during imagewise exposure, can influence the effect of oxygen inhibition and the rate of the photochemical reaction/s that occur, and therefore can impact the formation of raised features, particularly the fine highlight dots, of the relief structure of the resulting printing form. A photosensitive element is exposed in many prior art exposure apparatuses in which the precursor is at an ambient temperature at the start and the precursor is at a temperature above ambient at the end of the exposure. Some exposure apparatuses counter the possible temperature increase of the photosensitive element by cooling the exposure bed on which the photosensitive element rests. Some examples of commercial flat-bed exposure devices having an exposure bed that is cooled are CYREL® 1000ECLF, 1000ECDLF, DF1000ECLF, and DF2000EC sold by DuPont (Wilmington, Del., USA); Concept 302ec, 302eclf, 302ecdlf, 305edlf, 400ec, 400eclf, and 501ec sold by Glunz & Jensen (Ringsted, Denmark); and, Nyloflex Next Exposure FII, Next Exposure FV, Exposure unit FIII, Exposure unit FIV, Exposure unit FV, and Combi FIII sold by Flint (Luxembourg, Luxembourg). But even with the photosensitive element supported on a cooled exposure bed, the photosensitive element will still experience temperature changes during exposure. At the start of exposure the bed will be cool and the plurality of lamps will not have had sufficient time to warm the photosensitive element; and as the exposure progresses, the lamps will warm the photosensitive element but the cooled bed will mitigate significant increase in the temperature of the photosensitive element.
Problems can also arise with the quality and uniformity of the radiation emitting from each of the lamps and thus impinging the photosensitive element for a single exposure, as well as for one exposure of a photosensitive element to another exposure of a different photosensitive element, i.e., for exposures of multiple photosensitive elements over a period of time, particularly over the lifetime of the lamps. During exposure, the radiation impinging the photosensitive element should be evenly distributed over the area of the exposure bed, so that the entire exposed surface of the photosensitive element is uniformly irradiated. The plurality of light tubes when energized typically generates heat, which particularly in an enclosed environment interior to the exposure apparatus can influence the temperature of the lamps. So much heat may be generated by the lamps that the lamps overheat, and it can become difficult to maintain the lamps at a constant temperature or within a desired temperature range. It is desirable to maintain the temperature constant from lamp to lamp within the plurality of light tubes, as well as along the axial length of each of the lamps, and avoid relatively hot or cool regions in the lamps and from lamp to lamp. Barral et al. in U.S. Pat. No. 5,983,800 describe a machine for insolating photopolymer plates to ultraviolet light that delivers a flow of cooling air to the machine top in the vicinity of a negative, the photopolymer plate and a transparent membrane used to draw vacuum; and includes a bank of fans or blowers to provide some cooling of the bank of ultraviolet lamps. However, not all lamps of the bank of lamps are equally cooled by merely blowing air across the lamps to cool the lamps since relatively hot or cool regions can occur in the lamps and from lamp to lamp. Non-uniform lamp temperatures will generate non-uniformity in the irradiance of the radiation emitting from the lamps and thus impinging the photosensitive element.
Another problem is that lamps age with use, i.e., the irradiance emitted by a lamp or its intensity diminishes as the lamp is used. An integrator can be used to compensate for lamp aging to a certain degree, but either the exposures become too long or is insufficient to provide desired degree of photochemical reaction in the photosensitive element. Exposure apparatuses for photosensitive elements are known to have a radiation integration system, sometimes referred to as an integrator, which evaluates the intensity of the lamps illuminating the exposure bed where the printing form lies. An example of an exposure unit having an integrator is the CYREL® 1000ECLF. The integration system compensates the time of exposure according to the intensity of the radiation emitted by the lamps. The system may include a photocell that senses the radiation incident thereon, and a circuit that integrates a signal from the photocell. The photocell in these exposure units typically measures the intensity of the lamps for a broad spectrum of wavelengths of the emitted radiation. Such exposure control systems, should in theory provide the designed for exposure values. In practice however, since exposure is a function of many variable factors, there is potential for the exposure values actually produced by any such system to vary from the values designed for. Ultimately the integration of the lamp energy is not sufficient and all the lamps will need to be removed and replaced with new lamps. Even when the lamp or lamps are replaced, the light intensity drops off in the first 20 hours of lamp life, so that recalibration is necessary throughout this initial age-in of the lamps. Factors which affect the replacement of lamps are the physical location of the lamps within the hood, their elapsed operation time, and the elapsed operation time of all adjacent lamps. Frequent recalibration due to lamp replacement is an undesired step that can consume considerable platemaking time and manpower, as well as printing forms. Due to the problems and costs associate with lamp changes it is desirable to extend the life of the lamps without sacrificing uniformity and consistency in irradiation emitted by the lamps during exposure.
Therefore, there is a need to overcome the problems of related art and to provide an improved exposure apparatus and method for preparing relief printing forms from photosensitive elements using the exposure apparatus. It is desirable to insure proper exposure of photosensitive elements consistently over the useful life of the lamp/s in an exposure unit. It is also desirable to avoid the time, manpower, and materials associated with lamp replacement and recalibration of exposure unit to determine appropriate exposure for photosensitive elements. It is also desirable to extend the lifetime of the lamps without sacrificing the quality and uniformity of the exposure radiation. It is also desirable to insure proper exposure of photosensitive elements necessary to achieve satisfactory resulting relief structures for printing forms. There is a need to establish and maintain conditions in an exposure apparatus that can provide sufficiently consistent quality and uniformity of actinic radiation impinging photosensitive elements.